"Doctor, I've been in world where you died, 'member?" Rose says, her back still to him. She's leafing through her book more to give her shaking hands something to do. "Selfishly, I don't want to lose you. Ever. You're supposed to live so much longer than me. You're supposed to remember me when I'm dust in my grave. Outside of all my selfish reasons for wanting you to live, there's also the rest of the Universe. You make it a better place. You save us all. And if you save us all then you save other worlds too. If you die, the entire human race becomes extinct. After that, I'm guessing everyone except the Daleks."